This invention relates to a method of and plant for air separation. It is particularly concerned with such a method and plant in which the air is separated into an oxygen product and a nitrogen product, and part of the nitrogen product is supplied at an elevated pressure to a gas turbine.
GB-A-2028 991 relates to such a method and plant. A double rectification column is employed to separate the air. (A double rectification column has a higher pressure rectification column, a lower pressure rectification column and a condenser-reboiler placing an upper, usually a top, region of the higher pressure rectification column, in heat exchange relationship with a region, usually a bottom region, of the lower pressure rectification column.) The air is rectified in the higher pressure rectification column, to form an oxygen-enriched liquid fraction and a first vaporous nitrogen fraction. A stream of the oxygen-enriched liquid fraction is withdrawn from the higher pressure rectification column and is used to form a feed stream to the lower pressure rectification column so as to form an oxygen product fraction and a second vaporous nitrogen fraction. At least one stream of a nitrogen product is taken from the double rectification column. A part of the nitrogen product is raised in pressure and is introduced into a gas turbine comprising an air compressor, a combustion chamber which has a first inlet communicating with the air compressor and a second inlet communicating with a source of fuel, and an expander communicating with the combustion chamber for expanding the hot gaseous products of the combustion of the fuel. The nitrogen is introduced into the combustion chamber or the expander normally for the purpose of reducing emissions of oxides of nitrogen in the exhaust of the expander. The work done by the expander is typically used to generate electrical power.
Because the combustion chamber of the gas turbine normally operates at a high pressure, typically in the range of 10 to 20 bar, GB-A-2028991 discloses that downstream of being warmed to ambient temperature a stream of the second vaporous nitrogen fraction is compressed to the necessary high pressure upstream of its being introduced into the gas turbine. The nitrogen feed to the gas turbine is normally formed exclusively of the second vaporous nitrogen fraction, that is the nitrogen fraction separated in the lower pressure rectification column. In order to reduce the work that has to be done in compressing this nitrogen, GB-A-2 028 991 recommends operating the lower pressure rectification column not at its normal pressure in the range of 1 to 2 bar (absolute), but instead at a higher pressure typically in the range of 3 to 5 bar. Several disadvantages arise. First, the higher pressure rectification column now has to be operated at a pressure in the order of 8 to 12 bars rather than at a conventional pressure in the range of 5-6 bar. Therefore, more work needs to be performed in compressing the incoming air for separation, and more compression equipment is required, than when the higher pressure rectification column is operated at its conventional pressure. Second, increasing the operating pressures of the higher and lower pressure rectification columns reduces the volatility of nitrogen relative to oxygen. An increase in the number of distillation stages required to effect the separation results. Third, the amount of co-produced nitrogen at a pressure above atmospheric is fixed by the oxygen/nitrogen ratio in the feed air. It is very often the case that the amount of co-produced elevated pressure nitrogen is in excess of the requirements for control of NO.sub.x , emissions. There are therefore penalties in terms of thermodynamic efficiency to mixing all the nitrogen with the fuel gas.
It is an aim of the present invention to make it possible to provide a method and plant which make it possible to reduce these disadvantages, but not at the cost of significant additional work of compression of nitrogen compared with when the low pressure column is operated at the optimum pressure disclosed in GB-A-2 028 991 A.